High-purity silicon is required for a number of applications, including for example the production of Si.sub.3 N.sub.4 from silicon powder for high-purity ceramics or even in photo-voltaics. Nowadays, highly pure but also expensive semiconductor grade silicon is mainly used for such applications.
To obtain greater economy, inexpensive processes for the production of pure silicon are currently being developed worldwide. The most widely developed processes are based on the purification of inexpensive but impure silicon which is obtained in large quantities by carbothermal reduction of quartz, on the production of pure silicon by the use of prepurified carbon and quartz in carbothermal reduction and on the aluminothermal reduction of silicon tetrachloride with aluminium.
In every case, additional purification steps are necessary to reduce the excessive foreign atom contents of boron, phosphorus, carbon, metals, metal oxides and oxygen to concentrations in the ppm range.
Processes by which these impurities can be reduced are already known. To reduce the high contents of silicon dioxide and carbon mainly present in the form of silicon carbide in the silicon obtained by carbothermal reduction of quartz, German 3,411,955 and European 160,294 describe processes in which molten silicon is filtered through filters of graphite or a compound material of SiC/Si, the solid constituents remaining behind in the filter. This method is not satisfactory in economic terms because the filter layer becomes blocked in operation so that the purification process has to be interrupted. To overcome the blockage, the reactor has to be cooled from around 1420.degree. C., cleaned and the filter material dicarded.
In addition, German 3,403,131 describes a process in which silicon is melted in a graphite crucible so that the non-reduced quartz and the unreacted carbon collect on teh walls of the crucible. In continuous operation, accumulations of slack build up on the walls of the crucible so that the crucible ultimately becomes unuseable, incurring high costs.
German publisched patent applications 3,416,559 and 3,303,691 describe processes in which solid SiC and SiO.sub.2 impurities are removed from silicon melts by centrifugation and sedimentation.
Processes for removing dissolved impurities from molten silicon, in which the melt is exposed to the action of various gases, are described in the patent literature (see: German 2,623,413, German 2,929,089, European 7063, German 3,504,723, Brazil 83/6289, U.S. Pat. No. 4,312,849, France 2, 465,684, U.S. Pat. No. 4,298,423, and German 2,944,975).
Although individual foreign atoms can be removed by these processes, all foreign atom concentrations cannot be simultaneously reduced to the level required for solar cells. In particular, metal oxides, particularly aluminium oxide, cannot be economically removed by these processes. Another disadvantage of these processes is that they cannot be carried out continuously.
Accordingly, the object of the present invention is to provide an economic process for refining silicon which is not attended by any of the disadvantages described above.
A process which satisfies these requirements in d highly economical manner has now surprisingly been found. This process is the subject of the present invention.